The sense of hearing is fundamental to man's ability to communicate, and is particularly important in children as they acquire language. The sensory receptor cells of the inner ear play a pivotal role: their absence due to birth defects (congenital deafness), or their loss later in life, results in total deafness. To combat congenital deafness and the resulting deficits in human communication, it is necessary to understand the normal development of the inner ear. As the ear develops, the cells that comprise the ear must undergo a series of key decisions as to what cell type(s) to become (i.e., fate determination), and what spatial arrangements they must make with neighboring cells to generate the appropriate three-dimensional structure (i.e., pattern formation). The specific aims of this project are (1) to describe the kinds of cell fate decisions made by individual progenitor cells in the vertebrate inner ear through lineage analysis and (2) to test one candidate molecule for a role in cell fate determination and/or pattern formation. The methods for accomplishing these two specific aims primarily involve using retrovirus vectors as a highly efficient gene transfer technique to introduce genes into the developing inner ear of the chicken. For Specific Aim 1 (lineage analysis), replication-defective retrovirus vectors will be used to deliver a marker gene encoding either alkaline phosphatase or beta- galactosidase to individual progenitor cells as a way of labeling them and all of their progeny. Clonal analysis will be performed at a stage when regional and cell type distinctions are readily apparent. For Specific Aim 2 (gene perturbation), replication-competent retrovirus vectors that encode various forms of a homeobox-containing gene, SOHo-1, will be used to misexpress this protein in inner ear progenitor cells. SOHo-1 is proposed to play a role in the cell fate decision involving whether or not to become a sensory organ based on preliminary descriptions of its mRNA expression pattern. The purpose of this study is to further define the normal expression pattern of SOHo-1, and to alter this expression pattern during critical stages of ear formation. Alterations in the formation of the ear as a result of SOHo-1 misexpression would indicate that this gene does indeed play a functional role in establishing the normal anatomy of the ear. The gene perturbation methods established during the course of this study can be used in the future to test many other candidate genes for a role in ear development. The phenotypes obtained from gene perturbation experiments may mimic, and in some cases correlate with, human congenital malformations of the inner ear.